Dielectric composition on the basis of barium titanate

ABSTRACT

The invention relates to the dielectric composition on the basis of barium titanate (BaTiO 3 ) that is present with Ba(Zn 1/3 Nb 2/3 )O 3  in a perovslite structure, which dielectric composition exhibits an average grain size d 50  in the range of 0.2 μm to 0.5 μm and a crystallite size d 10  below 0.3 μm.

The invention relates to a dielectric composition on the basis of bariumtitanate (BaTiO₃) that is present with Ba(Zn_(1/3)Nb_(2/3))O₃ in aperovskite structure.

Dielectric compositions on the basis of barium titanate constitute animportant starting material for components used in electronics andtelecommunications. They are used to make passive electronic components,such as capacitors. For this purpose, the dielectric compositions mustexhibit a high dielectric constant K at ambient temperature of 2,000 ormore, values in excess of 5,000 being attainable.

An important technology for the manufacture of microelectronicsubstrates having a high packing density is the so-termed “LowTemperature Cofired Ceramics” technology, hereinafter referred to asLTCC technology. In accordance with said technology, insulating ceramicfoils that customarily also contain apertures for the verticalconnections are printed with metal line patterns, after which thelaminated foils are fired at temperatures in the range between 850° C.and 900° C. If known dielectric compositions are used, however, thefiring or sintering temperature must be increased considerably in orderto ensure that the sintering process is completed in a satisfactorymanner.

U.S. Pat. No. 4,283,753 discloses a dielectric composition having a highdielectric constant that is aimed to be sinterable at low temperatures.By specific monitoring of the stoichiometry and the charge equalizationit is achieved to provide a dielectric composition on the basis ofbarium titanate which can be sintered at temperatures of approximately1100° C. This comparatively low sintering temperature enables conductortracks to be used which are composed of a palladium-silver alloycomprising at least 30% by weight silver.

Particularly for high-frequency applications in the radio communicationtechnology, the metallic conductor tracks in modules produced inaccordance with the LTCC technology must be composed of a highlyconductive metal, preferably pure silver. Due to silver's low meltingpoint of 961° C., the sintering temperature of the ceramic massesshould, where possible, not exceed a value of 920° C. In addition, it isimportant for the ceramic masses to exhibit similar coefficients ofthermal expansion τ.

It is an object of the invention to provide a dielectric composition ofthe type mentioned in the opening paragraph, which enables the sinteringtemperatures to be so low that pure silver can be used as the materialfor the metallic conductor tracks.

For a dielectric composition on the basis of barium titanate that ispresent with Ba(Zn_(1/3)Nb_(2/3))O₃ in a perovskite structure, thisobject is achieved in that said dielectric composition has an averagegrain size d₅₀ in the range of 0.2 μm to 0.5 μm and a crystallite sized₁₀ below 0.3 μm. Preferably, the average grain size d₅₀ lies in therange of 0.3 μm to 0.4 μm, and the crystallite size d₁₀ is below orequal to 0.2 μm. It has surprisingly been found that the grain size andthe crystallite size of the base material used is of decisive importancefor the reduction of the sintering temperature to below 920° C.

To additionally modify the coefficient of expansion τ of the dielectriccomposition, in a preferred embodiment, sintering additives are usedthat required an inventive choice to be made since the sinteringadditives that are customarily used lead to a dramatic reduction of thedielectric constant. Said sintering additives also enable the sinteringtemperature to be further modified.

The sintering additives that can suitably be used for the dielectriccomposition are oxide mixtures comprising at least zinc oxide (ZnO),boron oxide (B₂O₃), silicon oxide (SiO₂) and copper oxide (CuO). Thepreferred content of copper oxide, with respect to the quantity ofBaTiO₃—Ba(Zn_(1/3)Nb_(2/3))O₃ mixed crystal, lies in the range of 0.25wt. % to 1.5 wt. %.

It is further preferred that the dielectric composition comprises zincoxide and boron oxide as boralite (Zn₄B₆O₁₃). Boralite may have beenadded in a quantity of 1 wt. % to 5 wt. % with respect to the quantityof BaTiO₃—Ba(Zn_(1/3)Nb_(2/3))O₃ mixed crystal.

In accordance with an equally preferred embodiment, the sinteringadditive and hence the dielectric composition comprises titanium oxide(TiO₂) and lithium carbonate (Li₂CO₃). In such a mixture, lithiumcarbonate and silicon oxide may have been pre-reacted to form siliconsilicate (Li₂SiO₃). Said lithium silicate may be present in a quantityof 0.5 wt. % to 2.5 wt. % with respect to the quantity ofBaTiO₃—Ba(Zn_(1/3)Nb_(2/3))O₃ mixed crystal.

Zinc oxide, silicon oxide and titanium oxide may have been pre-reactedto form ZST (ZnSiTiO₅); ZST may be present in a quantity of 1 wt. % to 5wt. % with respect to the quantity of BaTiO₃—Ba(Zn_(1/3)Nb_(2/3))O₃mixed crystal.

To increase the insulation resistance IR and the service life,preferably small quantities of manganese (Mn) are added to the mixedcrystal, as a result of which a base materialBa(Ti_(1-x-y)Mn_(y)[Zn_(1/3)Nb_(2/3)]_(x))O₃ is obtained, wherein0.03≦x≦0.1 and 0.001≦y≦0.01.

A method of manufacturing a BaTiO₃—Ba(Zn_(1/3)Nb_(2/3))O₃ mixed crystalfor a dielectric composition as described above comprises the followingsteps:

-   -   providing starting materials for barium, zinc, titanium, niobium        and, if necessary, manganese in pulverized form;    -   mixing the appropriate molar quantities of the starting        materials in an aqueous suspension;    -   drying the suspension and deaggregating the dried mixture to a        powder;    -   calcining the powder;    -   grinding the calcined powder to a grain size d₅₀ in the range of        0.2 μm to 0.5 μm so that the crystallite size d₁₀ is smaller        than 0.3 μm.

“In pulverized form” is to be taken to mean herein that the startingmaterials, in so far as they are solids, have been pulverized to grainsizes below 0.5 μm or are available in dissolved form or as colloids orthe like.

A method of manufacturing a dielectric composition that is compatible,in respect of sintering temperature and thermal coefficients ofexpansion, with commercially available glass ceramic layers comprisesthe following steps:

-   -   providing a powder of BaTiO₃—Ba(Zn_(1/3)Nb_(2/3))O₃-mixed        crystal, if necessary doped with manganese, having an average        grain size d₅₀ in the range of 0.2 μm to 0.5 μm, preferably in        the range of 0.3 μm to 0.4 μm, and a crystallite size d₁₀ below        0.3 μm, preferably below or equal to 0.2 μm;    -   providing a sintering additive having an average grain size        d₅₀<0.5 μm, which is selected from:    -   a) a mixture of zinc oxide, boron oxide, silicon oxide and        copper oxide,    -   a1) a mixture of boralite silicon oxide and copper oxide,    -   b) a mixture of zinc oxide, boron oxide, silicon oxide, copper        oxide, titanium oxide and lithium carbonate, or    -   b1) a mixture of boralite ZST, lithium silicate and copper        oxide, in a quantity of 1.5 wt. % to 15 wt. % with respect to        the quantity of BaTiO₃—Ba(Zn_(1/3)Nb_(2/3))O₃ mixed crystal;    -   mixing BaTiO₃—Ba(Zn_(1/3)Nb_(2/3))O₃ mixed crystal with the        selected sintering additive, making sure that the average grain        size is maintained.

By means of the dielectric composition in accordance with the invention,it is possible to manufacture a sintered laminated component comprisingat least two layers of a glass ceramic between which a dielectriccomposition in accordance with the invention is arranged, and comprisingintegrated electrodes of silver (Ag) or a silver-containing alloy.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter.

In the drawings

FIG. 1 graphically shows the temperature dependence of capacitances andlosses of a disk-shaped sample when use is made of a dielectriccomposition in accordance with the invention;

FIG. 2 shows a laminated component composed of two glass ceramic layersHereus AHT01-004 and a central layer of a dielectric composition inaccordance with the invention after sintering.

EXAMPLE 1

For the manufacture of the BaTiO₃—Ba(Zn_(1/3)Nb_(2/3))O₃ mixed crystal(base material) use is made of the following starting materials:

-   -   Ba: BaCO₃, ultrafine, d₅₀=0.16 μm (Solvay BM040, Massa, Italy)    -   Zn: ZnCO₃, pulverized, d₅₀<0.5 μm (Merck, Darmstadt)    -   Ti: Ti-tetrabutylate (TBT; Ti[OC₃H₇]₄, hydrolyzed in H₂O to        colloidal TiO(OH)₂; (Dynamit-Nobel, Troisdorf)    -   Nb: niobium oxalate, aqueous solution (H. C. Starek, Goslar)

The doping material manganese was used in the form of Mn(NO₃)₂ (Merck,Darmstadt).

The aim is to obtain a base material of the compositionBa(Ti_(1-x-y)Mn_(y)[Zn_(1/3)Nb_(2/3)]_(x)O₃, where 0.03≦x≦0.1 and0.001≦x≦0.01. The optimum values for x and y were found to be x=0.07 andy=0.003.

The starting components were intensively mixed in the appropriate molarquantities in an aqueous suspension in a bail mill. Subsequently thesuspension was dried under a radiant heater and deaggregated in a ballmill so as to obtain a loose powder. Said powder was then calcined for12 hours at approximately 1000° C. in air. X-ray diffractrometricrecordings show a cubic perovskite phase with small quantities of acarbonatic (BaCO₃) secondary phase. The calcined base material wasreduced in size in a ball mill to a particle size of d₅₀≈0.3–0.4 μm. Thespecific surface area of the ground powder was approximately 5–6 m²/g,corresponding to a crystallite size d₁₀ of 0.2 μm or less.

EXAMPLE 2

To manufacture the sintering additives use was made of startingmaterials in commercially available form, such as B₂O₃, ZnO, CuO, Li₂CO₃(Merck, Darmstadt), TiO₂ (TM-3, Fuji Titanium, Japan), SiO₂ (ErosilOx-50, Degusa Hanau).

If the sintering additives should also include pre-reacted intermediateproducts, namely boralith, ZST and lithium silicate, then thesepre-reacted intermediate products are prepared by mixing the appropriatestarting materials in a ball mill after which they are calcined in airfor two hours. The calcining temperature being approximately 940° C. forboralith, approximately 900° C. for ZST and approximately 800° C. forlithium silicate.

Mixtures as listed in the following Table 1 were prepared:

TABLE 1 Percentage with respect to base Starting material material (wt.%) Optimal (wt. % Mixture A Zn₄B₆O₁₃ 1 . . . 5 2 SiO₂ 0.25 . . . 2.5 0.5CuO 0.25 . . . 1.5 0.5 Mixture B Zn₄B₆O₁₃ 1 . . . 5 2 ZnSiTiO₅ 1 . . . 52.7 CuO 0.25 . . . 1.5 0.5 Li₂SiO₃ 0.5 . . . 2.5 1

The sintering additives were ground to an average grain size d₅₀<0.5 μm.

Sinterable dielectric compositions were prepared by mixing the basematerial in a ball mill with the appropriate sintering additivesreferred to as mixture A and mixture B. For these compositions, in thetemperature range of 20° C. to 900° C., the coefficient of expansion τexhibits values of τ=10 . . . 11 ppm/K. Commercially available glassceramics have a coefficient of expansion τ=9 . . . 11 ppm/K. Thecoefficients of expansion match surprisingly well, so that sinteringdoes not lead to stress cracking.

The dielectric and ceramic properties of the dielectric composition inaccordance with the invention were tested in disk capacitors having athickness of 600 μm and a diameter of 6 mm. For this purpose, thepowdery dielectric compositions were compressed to disk-shaped greenbodies at a pressure of 10 KN and subsequently isothermally sintered fortwo hours in air at 900° C., 920° C. and 940° C. at a heating andcooling rate of 300 K/h. For the electrodes use was made of vapordeposited CrNi/Au layers. The to measurement of the dielectricproperties in the range of −50° C. to +150° C. at an alternating voltageof 1 V and a frequency of 1 kHz showed a broad maximum of the dielectricconstant at room temperature, with K being in the range of 2400–3400,dependent upon the sintering temperature of the samples. FIG. 1 showsthe results at a sintering temperature of 900° C.

The properties of all samples are summarized in Table 2:

TABLE 2 Sintering Sintering Sintering temperature: temperature:temperature: 900° C. 920° C. 940° C. Sintering additive mixture ADensity g/cm³ 5.72 5.76 5.81 Dielectric constant K 2510 2830 3380 Tan δ0.006 0.005 0.005 Sintering additive mixture B Density g/cm³ 5.64 5.665.7 Dielectric constant K 2850 3230 4820 Tan δ 0.006 0.006 0.007 δ =loss angleThe composition of the mixed crystal used isBa(Ti_(0.927)Ml₀₀₃[Zn_(1/3)Nb_(2/3)]_(0.07))O₃.

EXAMPLE 3

The finely ground base material with a selected sintering additive wasused to prepare an aqueous binder emulsion on the basis of polyvinylalcohol. By means of the “doctor blade” method the binder emulsion waspoured out and formed into ceramic green foils having a thickness of 40μm. These green foils were stacked between green foils of glass ceramicmaterial (Heraeus type AHT 01-004) having a thickness of 100 μm andlaminated by applying a low pressure of 300 bar at a temperature of 85°C. The green laminated body was heated at a rate of 60 K/h to atemperature of 450° C. at which the binder was burned out for 2 hours inair. Subsequently, the LTCC laminated bodies thus manufactured wereheated at a heating rate of 120 K/h to maximum temperatures in the rangeof 900° C. to 940° C. and sintered in air for a half hour.

FIG. 2 is a microscopic sectional view of a fracture-free monolithiclaminated component wherein the layers adhere well to one another andhardly show signs of interreaction.

The method in accordance with the invention enables a laminatedcomponent with electrodes of pure silver to be sintered in afracture-free manner at suitable temperatures in the range of 900° C. to920° C. If the process requires higher temperatures, for example 940°C., then Ag/Pd-98/2 electrodes can be produced in a fracture-freemanner.

The invention thus enables passive circuit elements such as filters,high-pass and low-pass filters and circuits for adapting impedance to beimplemented in a laminated structure, the passive electronic componentsthat are integrated including not only capacitors but also coils andother components.

1. A dielectric composition on the basis of barium titanate (BaTiO₃)that is present with Ba(Zn_(1/3)Nb_(2/3))O₃ in a perovskite structure,characterized in that said dielectric composition exhibits an averagegrain size d₅₀ in the range of 0.2 μm to 0.5 μm and a crystallite sized₁₀ below 0.3 μm.
 2. A dielectric composition as claimed in claim 1,characterized in that said dielectric composition exhibits an averagegrain size d₅₀ in the range of 0.3 μm to 0.4 μm, and a crystallite sized₁₀ below or equal to 0.2 μm.
 3. A dielectric composition as claimed inclaim 1, characterized in that said dielectric composition compriseszinc oxide (ZnO), boron oxide (B₂O₃), silicon oxide (SiO₂) and copperoxide (CuO).
 4. A dielectric composition as claimed in claim 3,characterized in that it comprises copper oxide in a content of 0.25 wt.% to 1.5 wt. % with respect to the quantity ofBaTiO₃—Ba(Zn_(1/3)Nb_(2/3))O₃ mixed crystal.
 5. A dielectric compositionas claimed in claim 3, characterized in that it comprises zinc oxide andboron oxide as boralite (Zn₄B₆O₁₃).
 6. A dielectric composition asclaimed in claim 5, characterized in that it comprises boralith in aquantity of 1 wt. % to 5 wt. % with respect to the quantity ofBaTiO₃—Ba(Zn_(1/3)Nb_(2/3))O₃ mixed crystal.
 7. A dielectric compositionas claimed in claim 3, characterized in that it comprises titanium oxide(TiO₂) and lithium carbonate (Li₂CO₃).
 8. A dielectric composition asclaimed in claim 6, characterized in that lithium carbonate and siliconoxide are pre-reacted to lithium silicate (Li₂SiO₃).
 9. A dielectriccomposition as claimed in claim 8, characterized in that it compriseslithium silicate in a quantity of 0.5 wt. % to 2.5 wt. % with respect tothe quantity of BaTiO₃—Ba(Zn_(1/3)Nb_(2/3))O₃ mixed crystal.
 10. Adielectric composition as claimed in claim 6, characterized in that zincoxide, silicon oxide and titanium oxide are pre-reacted to ZST(ZnSiTiO₅).
 11. A dielectric composition as claimed in claim 10,characterized in that it comprises ZST in a quantity of 1 wt. % to 5 wt.% with respect to the quantity of BaTiO₃—Ba(Zn_(1/3)Nb_(2/3))O₃ mixedcrystal.
 12. A dielectric composition as claimed in claim 1,characterized in that manganese (Mn) has been added to said dielectriccomposition to form Ba(Ti_(1-x-y)Mn_(y)[Zn_(1/3)Nb_(2/3)]_(x))O₃, where0.03≦x≦0.1 and 0.001≦y≦0.01.
 13. A method of manufacturing aBaTiO₃—Ba(Zn_(1/3)Nb_(2/3))O₃ mixed crystal for a dielectric compositionas claimed in claim 1, comprising the following steps: providingstarting materials for barium, zinc, titanium, niobium and, ifnecessary, manganese in pulverized form; mixing the appropriate molarquantities of the starting materials in an aqueous suspension; dryingthe suspension and deaggregating the dried mixture to a powder;calcining the powder; grinding the calcined powder to a grain size d₅₀in the range of 0.2 μm to 0.5 μm so that the crystallite size d₁₀ issmaller than 0.3 μm.
 14. A method of manufacturing a dielectriccomposition comprising the following steps: providing a powder ofBaTiO₃—Ba(Zn_(1/3)Nb_(2/3))O₃-mixed crystal, if necessary doped withmanganese, having an average grain size d₅₀ in the range of 0.2 μm to0.5 μm, preferably in the range of 0.3 μm to 0.4 μm, and a crystallitesize d₁₀ below 0.3 μm, preferably below or equal to 0.2 μm; providing asintering additive having an average grain size d₅₀ below 0.5 μm, whichis selected from: a) a mixture of zinc oxide, boron oxide, silicon oxideand copper oxide, a1) a mixture of boralith, silicon oxide and copperoxide, b) a mixture of zinc oxide, boron oxide, silicon oxide, copperoxide, titanium oxide and lithium carbonate, or b1) a mixture ofboralith, ZST, lithium silicate and copper oxide, in a quantity of 1.5wt. % to 15 wt. % with respect to the quantity ofBaTiO₃—Ba(Zn_(1/3)Nb_(2/3))O₃ mixed crystal; mixingBaTiO3—Ba(Zn_(1/3)Nb_(2/3))O₃ mixed crystal with the selected sinteringadditive, making sure that the average grain size is maintained.
 15. Asintered laminated component comprising at least two layers of a glassceramic between which a dielectric composition as claimed in claim 1 isarranged, and comprising integrated electrodes of silver (Ag) or asilver-containing alloy.